Code completion for atoms

[mkremins]

Probably via om-autocomplete.

[lypanov]

In case it's useful: https://github.com/ninjudd/clojure-complete

[mkremins]

Just made an initial cut at the rendering code for completions: see commit 49486ffb186354945b626452a7bac964b93fcfbc.

Next steps:

• Actually set :completions on atom forms when appropriate.
• Somehow model the notion of a "selected completion" so that we can write commands that allow the user to select and insert a particular completion.
• Reflect the completion selection state in the UI.

[mkremins]

Commit 9fd33e3e9f14557221cb9b48d8873e4f07104e21 does most of this. Next steps:

• Consider hiding the completions menu by default, only exposing it (1) when the selected atom node is a placeholder and (2) when the user is actively editing the selected atom node.
• Include clojure.core defs and top-level defs established within the document currently being edited in the list of available completions.
• Discard duplicates from the list of available completions.
• Filter and sort available completions by similarity to the atom node's current text.
• Hide the completions menu when the position of the currently edited form prevents us from making good predictions about what the user might want to type (e.g. in (def |...|), there's no good way for us to guess what the user wants to name the var, so not suggesting anything is preferable).